Molecular Mechanisms by which Rexinoids Prevent BRCA

描述（由申请人提供）：类维生素a在动物和人类中都是很有前途的化学预防剂，但由于天然类维生素a具有相对毒性，因此通常不用于癌症预防。在之前的资助期间，我们证明了9-顺式维甲酸（9cRA），结合PAR和RXR类视黄酸受体，抑制转基因小鼠的乳腺肿瘤发生，但毒性太大，无法进一步开发为化学预防剂。然后，我们证明了像LGD1069这样的类维生素a选择性结合RXR（“类维生素a”）仍然可以抑制乳腺细胞的生长和肿瘤发生，并且毒性很小。这些临床前研究使我们能够设计和开展第一个使用rexinoids的人类癌症预防临床试验，目前正在NIH的单独资助下进行。我们还发现，rexinoids主要通过G1细胞周期阻滞抑制乳腺细胞生长。9eRA和LGD1069抑制肿瘤发生的能力依赖于特定的致癌途径——两者在通过过度表达bb2而发展为乳腺癌的小鼠中比在通过表达SV40 T抗原使Rb和P53失活的小鼠中更有效——但是，与他莫昔芬不同的是，两者都成功地阻止了er阴性和er阳性肿瘤。我们还发现，成功抑制小鼠乳腺肿瘤发生与乳腺上皮组织中增殖减少和Cox-2表达减少有关，这表明这既是预防肿瘤的生物标志物，也是未来治疗的潜在靶点。我们在此假设rexinoids通过调节生长调控抑制乳腺癌的发生。通过与特定结合伙伴的RXR异源二聚体在正常乳腺细胞中的表达，以及将类黄酮与选择的互补化学预防药物联合使用将更有效地预防乳腺癌。为了验证这些假设，我们将：(1)通过定义g1细胞周期阻滞的分子基础，并确定rexinoids如何调节关键生长调节剂（cyclin D1， IGFBP-6和Cox-2），阐明rexinoids抑制正常乳腺细胞生长的机制。(2)利用共免疫沉淀和酵母双杂交技术，鉴定介导rexinoids抑制肿瘤和抑制生长作用的关键RXR二聚化伙伴。然后在乳腺细胞中选择性地抑制单个RXR伴侣，以确认它们在类维生素a抑制生长中的作用。(3)在发生er阴性乳腺癌的转基因小鼠中，确定Cox2抑制剂与rexinoid联合使用是否比单独使用任何一种药物更有效地预防乳腺癌，以及这些药物联合使用是否增加了特定生物标志物的调节和增殖抑制。这些研究将为制定预防人类er阴性乳腺癌的有效策略奠定基础。

DESCRIPTION (provided by applicant): Retinoids are promising chemopreventive agents in animals and in humans, but because natural retinoids are relatively toxic, they are not generally used for cancer prevention. During the previous funding period we demonstrated that 9-cis retinoic acid (9cRA), which binds both the PAR and RXR retinoid receptors, suppresses mammary tumorigenesis in transgenic mice, but is too toxic for further development as a chemopreventive agent. We then demonstrated that retinoids like LGD1069 that selectively bind RXR ("rexinoids") still inhibit breast cell growth and tumorigenesis, with minimal toxicity. These preclinical studies enabled us to design and open the first human cancer prevention clinical trial using rexinoids, now ongoing under a separate NIH grant. We also showed that rexinoids suppress breast cell growth predominantly via a G1 cell cycle block. The ability of both 9eRA and LGD1069 to suppress tumorigenesis was dependent on the specific oncogenic pathway--both were more effective in mice that develop breast cancer through overexpression oferbB2 than in mice that have Rb and P53 inactivated by expression of SV40 T antigen--but, unlike tamoxifen, both successfully prevented ER-negative as well as ER-positive tumors. We have also found that successful suppression of mammary tumorigenesis in the mice is associated with reduced proliferation and reduced Cox-2 expression in the mammary epithelial tissues, suggesting both biomarkers of prevention and potential targets for future therapies. We here hypothesize that rexinoids suppress breast carcinogenesis by modulating growth regulatory. proteins in normal breast cells, via RXR heterodimers with specific binding partners, and that combining rexinoids with selected complementary chemopreventive agents will more effectively prevent breast cancer. To test these hypotheses, we will: (1) Elucidate the mechanism by which rexinoids suppress the growth of normal breast cells, by defining the molecular basis for the G l cell cycle block and determining how rexinoids modulate critical growth regulators (cyclin D1, IGFBP-6, and Cox-2). (2) identify the critical RXR dimerization partners that mediate the tumor suppressive and growth suppressive effects of rexinoids, using co-immunoprecipitation and yeast-two-hybrid techniques. Individual RXR partners will then be selectively suppressed in breast cells to confirm their role in growth inhibition by rexinoids. (3) Determine whether combining a Cox2 inhibitor with a rexinoid more effectively prevents breast cancer than either agent alone in transgenic mice that develop ER-negative mammary cancer, and whether the combination of these agents increases modulation of specific biomarkers and inhibition of proliferation. These investigations will provide a foundation for development of an effective strategy to prevent ER-negative breast cancer in humans.